1. Field of the Invention
The present invention relates to a method of forming oxide layers, and more particularly, to a method of forming oxide layers different in thickness each other on an identical layer.
2. Discussion of Related Art
In general, a flash memory device needs oxide layers which are different in their thickness. That is, the flash memory device includes high-voltage operational elements and low-voltage operational elements in accordance with an operation voltage employed therein. The high-voltage operational elements are necessary to be associated with thick oxide layers, while the low-voltage operational elements are necessary to be associated with thin oxide layers. Such needs for different oxide layers dependent on the operation voltage brought a dual-gate oxidation process, which processing steps are as follows.
FIGS. 1A through 1C are sectional views illustrating a method of forming oxide layers in a semiconductor device, in accordance with a conventional technique.
Referring to FIG. 1A, a first oxide layer 102 is formed on a semiconductor substrate 101 where it is comparted into a field for memory cells and low-voltage operational elements (hereinafter, referred to as “first field”) and a field for high-voltage operational elements (hereinafter, referred to as “second field”). The first oxide layer 102 is formed to make a gate oxide layer of the high-voltage operational element.
Referring to FIG. 1B, after forming an etch-stopping layer 103 on the second field, the first oxide layer 102 is removed from the first field. During this, it is general for the etch-stopping layer 103 to be formed by means of a photo-resistive pattern.
Meantime, the first oxide layer 102 on the first field can be removed by an etching process using a BOE (Buffered Oxide Etchant).
Referring to FIG. 1C, the etch-stopping layer 103 is also removed. Thereafter, a second oxide layer 104 is deposited on the first field. While this, it is occurred that the second oxide layer 104 is partially formed on the first oxide later 102 of the second field. The second oxide layer 104 is formed to make a tunnel oxide layer of a memory cell or a gate oxide layer of the low-voltage operational element.
From the processing steps, the first and second oxide layers are formed with different thickness each other.
However, with such a method for forming the oxide layers different in thickness, the surface of the semiconductor substrate 101 may be exposed when removing the first oxide layer as shown in FIG. 1A. During this, the BOE causes damages on the surface of the semiconductor substrate 101. Further, according to a longer processing time with the BOE, the surface of the semiconductor substrate 101 becomes harsher to may result in generating micro-trenches (not shown). As a result, the second oxide layer 104 degrades with its physical quality, causing electrical characteristics of the low-voltage operational elements to be worse.